Mixtures or pastes based on cellulose esters have earlier been proposed in EP-B-076 443 and EP-B-184 127.
The products described in these documents are used to a large extent for varnishing wood, metal, synthetic material, paper, leather, glass, and plastic films, all surfaces where great value is placed on solventless coating compositions.
A drawback to the coats obtained using the known coating compositions is their poor hardness and comparatively low resistance to various liquids such as alcohol, red wine, coffee, and the like.
The invention now provides a coating composition with markedly enhanced properties such as greater hardness and improved resistance to chemicals. The invention consists in that in the mixtures or pastes of the known type mentioned in the opening paragraph wherein at least a portion of the binder resin is functionalized with an amino-functional acetal. "Amino-functional" means an amino group is present.
It should be noted that it is known in itself from EP-B-255 608 that a thermosetting coating composition can be obtained by mixing hydroxyl group-containing compounds with a non-volatile acetal-functional cross-linking agent which can be obtained by converting a compound having a free isocyanate group with a hydroxyl-functional acetal. A possible drawback to the coating compositions described in this document is that when they are applied as coats, they have to be cured at a comparatively high temperature. A further drawback is the limited availability in industrial quantities of the hydroxyacetal used in these coating compositions.
EP-A-744 449 also describes mixtures or pastes based on cellulose esters and a binder resin in coating compositions. The use of a polysiloxane compound is required to achieve enhanced stability in water, while the use of an aqueous polyisocyanate dispersion as cross-linking is necessary to obtain a coating with markedly enhanced properties.
For that reason it must be considered extremely surprising that the now proposed coating compositions can be cured in a comparatively short time even at ambient temperature. This is of major significance for industrial coating processes of heat sensitive substrates like, e.g., wood which is used in the furniture industry. The use of coatings curable only at high temperatures would not only require unacceptably high investments in that branch of industry, but also result in higher costs (e.g. energy).